Electrophotographic (also called xerographic) copiers were first introduced in 1959. More recently the electrophotographic technology has been extended to printers. An excellent text on electrophotography is Electrophotography and Development Physics, by L. B. Schein (2d ed. 1996, Laplacian Press).
A typical modern electrophotographic ("EP") printer uses some sort of processor to interpret a program representing the image to be printed. The interpretation usually involves conversion of the program into a bitmap, which determines a pattern of light that will expose a photoreceptor, such as the surface of a drum or belt. A copier may operate in analog fashion by imaging light reflected from the document to be copied onto the photoreceptor or it may use digital data (acquired by digitizing the document) in the same manner as a printer.
For both copiers and printers, the exposure of the photoreceptor results in its acquiring a charge pattern. The photoreceptor then passes a toner dispenser and attracts toner particles to the areas corresponding to the charge pattern. The photoreceptor transfers the toner to a print medium such as a piece of paper. The toner is fused to the paper, usually with heat, and the paper exits the printer.
EP copiers and printers have been developed with a variety of design choices. One design choice is with respect to whether the photoreceptor surface is on a drum or belt. Another design choice is with respect to the paper transport mechanism, which may be a drum or a belt. Regardless of the configuration of the photoreceptor and the paper transport, at some point, toner from the photoreceptor is transferred to the paper. Sometimes, instead of transferring toner directly to paper, an intermediate drum or belt surface is used as an intermediate media. In theory, four possibilities exist for transferring toner from the photoreceptor to either the paper or intermediate media: drum-to-drum, drum-to-belt, belt-to-drum, or belt-to-belt.
The choice of belts versus drums is related to the configuration of the paper path. A straight paper path is thought to reduce the likelihood of paper jams. The benefits of a straight paper path with top-side access to the paper path have been successfully marketed for black and white printers.